Impact of Biomass Burning Organic Aerosol Volatility on Smoke Concentrations Downwind of Fires

Abstract

Biomass burning particulate matter (BBPM) affects regional air quality and global climate, with impacts expected to continue to grow over the coming years. We show that studies of North American fires have a systematic altitude dependence in measured BBPM normalized excess mixing ratio (NEMR; ΔPM/ΔCO), with airborne and high-altitude studies showing a factor of 2 higher NEMR than ground-based measurements. We report direct airborne measurements of BBPM volatility that partially explain the difference in the BBPM NEMR observed across platforms. We find that when heated to 40− 45 °C in an airborne thermal denuder, 19% of lofted smoke PM 1 evaporates. Thermal denuder measurements are consistent with evaporation observed when a single smoke plume was sampled across a range of temperatures as the plume descended from 4 to 2 km altitude. We also demonstrate that chemical aging of smoke and differences in PM emission factors can not fully explain the platformdependent differences. When the measured PM volatility is applied to output from the High Resolution Rapid Refresh Smoke regional model, we predict a lower PM NEMR at the surface compared to the lofted smoke measured by aircraft. These results emphasize the significant role that gas-particle partitioning plays in determining the air quality impacts of wildfire smoke.

Enhanced Light Absorption and Elevated Viscosity of Atmospheric Brown Carbon through Evaporation of Volatile Components

“…The volatility distributions and their mean values shown in Figure b demonstrate systematic changes across the PO x samples, indicating a shift from the mostly IVOC compounds in PO 1 to the much less volatile SVOC and even LVOC species in PO 3 , aligning with the removal of small volatile constituents in the PO 1 sample. Accumulation of low-volatility species in the progression of PO 1 to PO 3 samples aligns with the enhanced absorbance of these samples, as evidenced from Figure a, supporting the darkening-through-volatilization mechanism suggested in our previous study and corroborated by field observations. − …”

“…This may potentially enhance the actual MAC­(λ) values of aged aerosols, which may not have been fully captured in our evaporation experiments conducted on isolated, oily, flat surfaces. Regardless, the darkening trend observed in our experiments is in alignment with the phenomenon of darkening through volatilization observed in both laboratory experiments and field studies, − consolidating the validity of our results. Furthermore, aged BrC aerosols characterized by accumulated viscous, stronger absorbing less polar components, , present a challenge for chemical analysis using dissolution in common organic solvents.…”

“…It follows that darkening of BrC aerosol by evaporation takes place at a timescale shorter than the aerosol lifetime and its atmospheric multiphase reaction chemistry. Various field studies report that viscous and highly absorbing tar ball particles are commonly found in BB smoke plumes after prolonged atmospheric aging. − Our findings suggest that future observations of tar balls and their buildup in BB smoke plumes need to consider the combined effects of evaporation and reaction chemistry. In this context, it is important to note that the very high absorption coefficients reported for tar balls and other refractory particles, based on electron energy-loss spectra acquired under high vacuum conditions in electron microscopy studies, ,,,, are likely considerable overestimates with limited relevance to particles in real atmospheric conditions.…”

Secondary Organic Aerosol Formation from the OH Oxidation of Phenol, Catechol, Styrene, Furfural, and Methyl Furfural

“…In particular, as BB smoke ages and dilutes, the concentration of OA decreases from ∼2000 to 10 μg m −3 and less. 28,29 This aging and dilution process creates competition between dilution-driven evaporation of species from the aerosol and chemistry driven condensation of oxidized VOCs. 29−31 Therefore, in very concentrated smoke, intermediate volatility organic compounds (IVOCs) contribute most significantly to OA evaporation; and for less concentrated smoke, semivolatile organic compounds (SVOCs) contribute the most to it.…”

“…29−31 Therefore, in very concentrated smoke, intermediate volatility organic compounds (IVOCs) contribute most significantly to OA evaporation; and for less concentrated smoke, semivolatile organic compounds (SVOCs) contribute the most to it. 29 To accurately encapsulate the large OA range found during the wildfire aging process, a volatility basis set (VBS) 32 is used to represent SVOCs and IVOCs at wildfirerelevant OA. These values have not been previously reported for the species studied here, despite VBSs being the most common method for representing SOA in modern large-scale models.…”